This invention relates generally to a method of forming a high resolution pattern of transparent conductive tin oxide on a transparent vitreous substrate and, more particularly, to a process for forming such a pattern using a polycrystalline material on the substrate in mirror image to the pattern sought to be formed and using a high temperature spray process for forming the tin oxide.
The electrical display industry is developing a variety of displays which require a transparent vitreous material having a transparent electrically conductive coating formed thereon. The transparent electrically conductive coating is generally formed in a conductive pattern for conducting electrical current from edge portions of the substrate to central locations defining the display area. In one embodiment, the transparent pattern defines portions of the display for activating liquid crystals or gas discharge displays located therebehind and forming the desired visible pattern. Generally, a highly transparent and electrically conductive layer is required.
The prior art has developed many transparent electrically conductive materials which can be formed in suitable patterns. A particular material is a thin film of tin oxide formed with a dopant to increase the electrical conductivity of the thin tin oxide film. However, conventional dopants are frequently not stable at high temperatures. Accordingly, it would be desirable to use an undoped film of tin oxide where the resulting film must be subjected to high temperatures, either in further processing or in final application.
However, it is difficult to form films of tin oxide having the requisite high conductivities since film formation generally requires the use of high temperatures to improve the adherence of the tin oxide to the transparent vitreous substrate. In a conventional technique using conventional photolithographic masking layers to form a negative image of the desired pattern, the tin oxide is formed in the exposed portions of the substrate. Conventional photolithographic materials do not perform well when subjected to the high temperatures required for a suitable tin oxide film and can even interfere with forming transparent conductive films, either by introducing impurities during the heating process or by degrading during heating to a degree that high resolution films are not possible.
One prior art solution to this problem, as shown in U.S. Pat. No. 4,009,061, is to form the film of tin oxide over the entire substrate surface and thereafter chemically etch the film to form the desired pattern. The tin oxide is relatively inert and a hot solution of metal in acid is required to reduce the selected portion of the tin oxide for removal.
Another prior art solution is depicted in U.S. Pat. No. 3,928,658, where a metallic layer is used to form the negative portion of the desired pattern. The selected metal is generally more easily removed than the overlying tin oxide when subjected to relatively weak solvent solutions. However, removal of the tin oxide layer requires the solvent to penetrate the tin oxide layer to the underlying metallic layer and this can be difficult. A conventional process employs a vacuum deposition system for forming both the substrate masking layer and the electrode area. The resulting film of tin oxide overlying the masking layer is relatively continuous and thereby resistant to solvent penetration for masking layer removal.
An undoped tin oxide film having a low electrical resistivity has been developed by Photon Power, Inc., as described in U.S. Pat. Nos. 3,880,633 and 3,959,565, and pending patent application Ser. No. 886,890, now U.S. Pat. No. 4,224,355. However, the tin oxide film is formed by a spray technique which requires temperatures near the softening point of glass in order to obtain the desired optical and electrical characteristics. Thus, using the prior art masking techniques hereinabove discussed is extremely difficult with respect to forming a high resolution conductive pattern.
In many applications it may be desirable to form two or more intersecting electrically conductive patterns which are electrically insulated from one another. In the above referenced conventional techniques, the masking material may be a metal which does not serve to insulate between the intersecting conductive strips or may be a conventional photolithographic masking material which degrades at high temperatures. Furthermore, the tin oxide film can be poorly adherent to materials and form crossing junctions which are easily damaged.
Yet another problem occurs where chemical etching is used. Under-etching occurs where portions of the tin oxide film are in contact with the etchant for longer than other portions of the tin oxide films, leading to a loss in resolution available for the circuit. The minimum spacing is controlled by the ability to control the etchant rather than the ability to deposit the films.
The disadvantages of the prior art are overcome by the present invention, however, where an improved process is provided for forming the electrically conductive pattern using a polycrystalline material to form the mirror image.